1. Field of the Invention
The present invention relates to two dimensional mapping of finger print patterns for identification purposes utilizing a capacitive circuit array.
2. Description of the Prior Art
Fingerprint sensing and the associated identification systems which include data bases and a match algorithm processor have been available in the prior art. In the area of the fingerprint sensor, the primary prior art systems utilizes an optical scanning method which is relatively bulky and expensive due to the optics, lasers and the CCD array utilized. Due to this reason, there have been attempts to develop electronic means of sensing fingerprint patterns. All electronic fingerprint sensors can be categorized as follows; tactile pressure sensors, thermal sensors and capacitive sensors. The first two categories are complex and expensive to make, and thus most of recent development activities are in the area of capacitive sensors.
One group of capacitive sensors rely on thin deformable membranes with metal electrodes (one electrode per pixel) coated underneath each membrane. If the membrane is very thin and can follow the finger surface deformation, the distance of the metal electrodes can be measured through capacitance means. However, such thin membranes are not durable, and hence, are not yet marketable.
Another group of capacitive fingerprint sensors read the capacitance from the rigid sensor electrodes to the finger surface ridges directly without relying on membranes. In this case, the capacitance variation due to the finger surface variation is minute, with a typical order to a few femto farads, and the signal is imbedded in the larger background and parasitic capacitance from the finger and the sensor structure. Therefore, somewhat complex circuitry such as those disclosed by Tartagni et al, in an article entitled xe2x80x9cFingerprint Sensor Based on the Feedback Capacitive Sensing Schemexe2x80x9d, IEEE Journal of solid State Circuits, Vol. 33, p. 133 (1/1998) and U.S. Pat. No. 5,835,141 to Aukland et al, have been implemented to filter out the background capacitance. The circuitry utilized in Tartagni et al, and Aukland et al, involve several transistors, an amplifier or charge accumulation and transfers in every pixel, making the sensor array essentially a large silicon chip and expensive to manufacture.
Copending application Ser. No. 09/550626 describes a new way of sensing with simple low cost fabrication by applying external fields which are diametrically opposed in respect to a pick up pad in the center. When a uniform finger surface is in contact, the fields cancel and pick up pad reads a null signal, but if there is a surface ridge, the fields are imbalanced and the pick up pad picks up a signal. Although the circuit described therein performs satisfactorily, it is desired to provide an improved system wherein the sensor is more sensitive and substantially immune from misreadings.
The present invention provides top electrode patterns applied on a substrate to which four or more phase external fields are applied at a given sensor scan time (frame time) or at two consecutive scan times. The readout lines and the sensor structure are symmetrical such that the parasitic capacitance from the sensor structure cancel out. The advantage of being able to fabricate the sensor on any substrate, such as glass, polymer and ceramics, is retained, the sensor being more sensitive and substantially immune from false readings.